This is a multicenter NCI chemoprevention seven year clinical trial using finasteride to reduce the incidence of prostate cancer in healthy men. Finasteride is a 5-alpha-reductase inhibitor which may play a pivotal role in the initialization and progression of prostate cancer. After an initial run-in period, volunteers without evidence of prostate enlargement and with a normal PSA will be randomized to receive either daily finasteride or matched placebo for 7 years or until development of prostate-related symptoms.